1. Field of the Invention
The present invention relates to an optical-writing apparatus, and more particularly to an optical-writing apparatus in which a plurality of LED (light emitting diode) array units are arranged in a staggered manner in a main scanning direction of a photoconductive member to expose respective regions of the photoconductive member.
2. Discussion of the Background
A background optical-writing apparatus employs an LD (laser-diode) scanning configuration or a light-emitting element array unit configuration, to write a latent image to a photoconductive member.
The LD scanning configuration includes a light source using a laser-diode to emit a laser beam, a polygon mirror, and a polygon motor that rotates the polygon mirror.
The light-emitting element array unit configuration includes a light source using light-emitting elements such as an LED (light emitting diode) to emit light, and the light-emitting elements are arranged in the light-emitting element array unit. Different from the LD scanning configuration, the light-emitting element array unit configuration can realize a higher reliability of its performance because it has no movable components such as a polygon mirror.
Furthermore, the light-emitting element array unit configuration requires no space for scanning a light beam to a main scanning direction that is required for the LD scanning configuration. Instead, the light-emitting element array unit configuration uses, for example, an LED array unit that integrates a plurality of optical elements including LEDs, self-focus lenses (e.g., a SELFOC lens manufactured by Nippon Sheet Glass Co., Ltd.). Thus, an image forming apparatus employing the light-emitting element array unit configuration can miniaturize its size.
Therefore, an image forming apparatus used for printing a wide format such as an A0-sized format has been adopting the light-emitting element array unit configuration instead of the LD scanning configuration. However, the light-emitting element array unit configuration needs an LED array unit that is longer than an image-writing width.
In a case of an image forming apparatus being used for printing a wide format such as an A0-sized format, a relatively long light-emitting element array unit that can cover the wide format such as an A0-sized format is required, resulting in an increase in the needed number of LED driver ICs (integrated circuits). Consequently, a production yield of the light-emitting element array unit decreases.
Furthermore, the relatively long light-emitting element array unit inevitably requires components that have higher precision than a relatively small light-emitting element array unit used for a relatively small image forming apparatus to secure a high precision level of writing to a photoconductive member. Consequently, the relatively long light-emitting element array unit becomes expensive. Furthermore, such an expensive light-emitting element array unit as a whole is needed to be replaced in an event that even a single LED is damaged in the light-emitting element array unit.
Accordingly, using a relatively long light-emitting element array unit is unfavorable from a viewpoint of overall cost.
In view of the above-mentioned drawbacks, a background technique uses a plurality of light-emitting element array units used for a relatively small image forming apparatus, and arranges them in a main scanning direction of an image forming apparatus used for printing a wide format such as an A0-sized format. For example, a background image forming apparatus arranges two or three light-emitting element array units including light-emitting elements such as an LED (light emitting diode) in parallel to an axial direction (i.e., main scanning direction) of a photoconductive member. The light-emitting element array units expose respective surface regions of the photoconductive member.
In a case of a background image forming apparatus used for printing a wide format such as an A0-sized format, three light-emitting element array units used for an A3-sized format are arranged in parallel to a main scanning direction in a staggered manner so that a total length of the three light-emitting element array units can cover a wide format such as an A0-sized format. The three light-emitting element array units expose respective surface regions of the photoconductive member in the main scanning direction. In this way, the image forming apparatus used for printing a wide format such as an A0-sized format is provided with a less expensive optical-writing apparatus.
In the above-mentioned configuration, a plurality of light-emitting element array units are arranged in a staggered manner in a main scanning direction while end portions of the light-emitting element array units overlap with each other. In a case of using three light-emitting element array units, an end portion of a first light-emitting element array unit and an end portion of a second light-emitting element array unit overlap with each other, and another end portion of the second light-emitting element array unit and an end portion of a third light-emitting element array unit overlap with each other. Such end portions become joint portions for the three light-emitting element array units. In each of the light-emitting element array units, a plurality of LEDs are consecutively arranged.
The LEDs in the joint portions also need to be arranged consecutively, that is an LED at the joint portion of the first light-emitting element array unit and an LED at the joint portion of the second light-emitting element array unit need to be consecutive with each other so that image data for one main scanning line are securely written to the photoconductive member at such a joint portion. Similarly, an LED at the joint portion of the second light-emitting element array unit and an LED at the joint portion of the third light-emitting element array unit need to be consecutive with each other so that image data for one main scanning line are securely written to the photoconductive member at such a joint portion. The above-mentioned LED at the joint portion is referred to an “overlaying LED” hereinafter.
When viewing the overlaying LEDs from a sub-scanning direction of the photoconductive member, an overlapping degree of the overlaying LEDs of adjacent light-emitting element array units is within a range of zero (i.e, the minimum degree) to one LED (i.e, the maximum degree). The overlapping degree “zero LED” means that two overlaying LEDs do not overlap with each other, and the overlapping degree “one LED” means that two overlaying LEDs completely overlap with each other.
In an actual image forming apparatus, two overlaying LEDs inevitably overlap with each other with some overlapping degree within a range of zero to one LED. Depending on an overlapping degree of the two overlaying LEDs, light intensity at the joint portion varies. Consequently, an image formed based on such light intensity may result in a poor quality image in some cases.
To compensate for an effect caused by the above-mentioned overlapping, a background technique uses a multiple-value method to control an emission of the overlaying LEDs by adjusting light intensity of the overlaying LEDs. For example, when an emission of the overlaying LEDs are controlled by image data expressed in five-bits, light intensity of the overlaying LEDs can be changed gradationally in 32 levels, and the above-mentioned effect caused by the above-mentioned overlapping can be eliminated.
However, when an emission of the overlaying LEDs is controlled by image data expressed in a binary format taking a value of “0” or “1,” light intensity of the overlaying LEDs cannot be changed gradationally, and the effect caused by the above-mentioned overlapping cannot be eliminated.
For example, as illustrated in FIG. 1, three LED array units 503_1 to 503_3 are arranged in an axial direction (i.e., the main scanning direction) of a photoconductive member in a staggered manner. Hereinafter the LED array units 503_1 to 503_3 are abbreviated as LAU 503_1 to 503_3. As illustrated in FIG. 1, the size of one LED represents one dot. A black dot means that an LED emits light and a white dot means that an LED does not emit light.
Hereinafter, for the clarity of explanation, four dots are designated as “a, b, c, and d” as illustrated in FIGS. 1 to 4.
To minimize the overlapping degree of overlaying LEDs, the LAUs 503_1 to 503_3 are positionally adjusted so that the overlaying dots “a” and “b” overlap with each other with an overlapping degree up to a full size of one dot.
FIGS. 2 to 4 illustrate examples of emitting conditions of dots arranged in the LAUs 503_1 to 503_3, which use the sample data sequence of “00110011” that repeats “00” and “11” in a toggle manner, and also illustrate light intensity waveforms corresponding to such a sample data sequence. In each example in FIGS. 2 to 4, an explanation is given by paying consideration to the dots “a, b, c, and d” that are placed consecutively.
FIG. 2 illustrates a case in which an overlapping degree of the overlaying dots “a” and “b” is relatively significant. As illustrated in FIG. 2, the dots “a, b, c, and d” take data of “1001”, wherein the value of “1” means that an LED emits light and the value of “0” means that an LED does not emit light. In this case, as illustrated in the light intensity waveform in FIG. 2, an interval formed by a down-edge of the overlaying dot “a” and a down-edge of the dot “c” is relatively smaller compared to other intervals shown in FIG. 2. Thus, the dot “a” and the dot “c” interfere with each other. Consequently, human eyes may perceive a black image formed around the dots “a, b, c and d” to be darker than a black image formed in an area not affected by the dots “a, b, c and d.”
FIG. 3 illustrates another case in which an overlapping degree of the overlaying dots “a” and “b” is relatively significant. As illustrated in FIG. 3, the dots “a, b, c, and d” take data of “1100.” In this case, as illustrated in the light intensity waveform in FIG. 3, a width of the light intensity waveform (i.e., black width) at the overlaying dots “a” and “b” becomes relatively smaller compared to other light intensity waveform areas not affected by the overlaying dots “a” and “b,” and thus black images corresponding to the overlaying dots “a” and “b” may shrink compared to other images not affected by the dots “a and b.” Consequently, human eyes may perceive a shrinked black image formed around the dots “a” and “b”.
FIG. 4 illustrates another case in which an overlapping degree of the overlaying dots “a” and “b” is relatively significant. As illustrated in FIG. 4, the dots “a, b, c, d” take data of “0011.” In this case, both of the overlaying dots “a” and “b” take a value of 0, and thus an emission at the dot “a” and “b” cannot be compensated in any way. Consequently, human eyes may perceive a black image formed around the dots “a” and “b” to be darker than a black image formed in an area not affected by the dots “a” and “b”.
The above-mentioned problem relating to the image quality is generally referred to as “black streak.”
As explained above, an image quality control at the overlapping portion using image data taking a binary format is substantially difficult.